I did an experiment this week. I decided to type only in QWERTY for 3 days (after not using it for 6 months).

Day 1: I had to look at the keyboard most of the time, speed ~30wpm, (my fastest stable speed was 70wpm, 6 months ago).

Day 2: I was able to type without looking at the keyboard, with a steady 35wpm.

Day 3: I was typing 45wpm without making typos due to my Colemak confusion.

Day 4: I changed back to Colemak. Started with this thread, when I began typing the 1st sentence I was making loads of mistakes due to QWERTY/Colemak confusion. Now at this point of, after only a couple of lines of typing, I'm already used to typing with Colemak once again at my previous speed, without errors.

Observation: When I was typing in QWERTY, I realized how much finger jumping around I had to do as opposed to while using Colemak.

Conclusion: My experiment was successful since before my hypothesis was that it takes a lot of practice to get back to typing in QWERTY after not using it for many months, but apparently it's very easy to get used to QWERTY once again, and attain the old speed, if it is needed.